Carrierless amplitude modulation/phase modulation (CAP) is a bandwidth-efficient two-dimensional passband line code. (Additional information on a CAP communications system can be found in J. J. Werner, "Tutorial on Carrierless AM/PM--Part I--Fundamentals and Digital CAP Transmitter," Contribution to ANSI X3T9.5 TP/PMD Working Group, Minneapolis, Jun. 23, 1992.) CAP is closely related to the more familiar quadrature amplitude modulation (QAM) transmission scheme. In voiceband modems, QAM has been used for over 25 years, while CAP has been used for over 15 years. However, CAP is simpler to implement digitally. Illustrative prior art transceiver structures for the QAM and CAP transmission schemes are shown in FIGS. 1 and 2, respectively. Both FIGS. 1 and 2 illustrate two-dimensional encoding where a complex symbol, A.sub.n, is applied to the transmitter portion (where A.sub.n =a.sub.n +jb.sub.n), and a recovered complex symbol, A.sub.n, is provided by the receiver portion, where A.sub.n =a.sub.n +jb.sub.n. With respect to other notation used in these FIGS., g(t) (e.g., see FIG. 1) is the impulse response of a baseband shaping filter, e.sub.in (t) and e.sub.qu (t) are equalizers for the in-phase and quadrature components, respectively, and p(t) and p(t) are impulse responses of a passband shaping filter which form a Hilbert pair (e.g., see FIG. 2).
To observe the difference between QAM and CAP transceivers, notice in FIG. 1 that the conventional QAM transmitter consists of a baseband pulse shaping filter followed by a modulator. The idealized QAM receiver for the case of no intersymbol interference (ISI) and additive Gaussian noise inverts these operations first using a demodulator and then a matched filter. An equivalent representation of the QAM transceiver is shown in FIG. 3, where the filtering and modulation operations have been reversed. This is known as the passband representation because the filtering is done at passband. To compensate for ISI, the matched filter has been replaced by an equalizer. Finally, removing the modulator and demodulator in FIG. 3 yields the CAP transceiver of FIG. 2, which attains the same theoretical performance as QAM but is simpler to implement digitally.
Currently, some broadband access applications, such as VDSL (Very high rate Digital Subscriber Line), may require either a CAP receiver or a QAM receiver. Some in the art have proposed simply putting both the CAP receiver and the QAM receiver into one receiver--in effect having a dual structure receiver with a CAP section (having its own equalizer) and a separate QAM section (with its own equalizer). To further complicate matters, this dual structure receiver may require the use of blind equalization techniques in both the QAM section and the CAP section. In this case, there is no training signal for the dual structure receiver to use to identify the type of modulation. As such, the dual structure receiver must first independently converge both the equalizer in the QAM section and the equalizer in the CAP section, and then make a decision as to the type of modulation before switching to the correct steady state operating mode.
An alternative approach is described in the above-referenced U.S. patent application of Werner et al., entitled "Blind Start-Up Of A Dual Mode CAP-QAM Receiver." In this approach, a receiver utilizes a single equalizer for supporting both a CAP mode of operation and a QAM mode of operation.